The reaction proceeds via a aldehyde
intermediate which then reacts with the second equivalent of the hydride reagent
(review)

Since the aldehyde is more reactive
than the ester, the reaction is not normally used as a preparation of aldehydes
.

MECHANISM OF THE REACTION
OF LiAlH4 WITH AN ESTER

Step 1:
The nucleophilic H from the hydride
reagent adds to the electrophilic C in the polar carbonyl group
of the ester. Electrons from the C=O move to the electronegative
O creating the tetrahedral intermediate a metal alkoxide
complex.

Step 2:
The tetrahedral intermediate collapses and displaces the alcohol portion
of the ester as a leaving group, in the form of the alkoxide, RO-.
This produces an aldehyde as an intermediate.

Step 3:
Now we are reducing an aldehyde (which we
have already seen)
The nucleophilic H from the hydride reagent adds to the electrophilic
C in the polar carbonyl group of the aldehyde. Electrons from the
C=O move to the electronegative O creating an intermediate
metal alkoxide complex.

Step 4:
This is the work-up step, a simple acid/base reaction. Protonation of
the alkoxide oxygen creates the primary alcohol product from the intermediate
complex.